[TSVB] Chart with override index pattern and last_value are migrated with entire_time_range from 7.13+

[stratoula]

Kibana version:
7.13+

Describe the bug:
On 7.13 we changed the default datarange mode from last_value to entire_timerange. #93608

It seems that when a chart was created before this version with:

• override_index_pattern to yes
• last_value

when is migrated to a 7.13+ kibana the datarange mode is not migrated and uses the default value

Steps to reproduce:

1. Create a metric TSVB chart (from 7.12 for example)
2. Override the index pattern from the options tab
3. Leave as datatange mode the last_value
4. Export the SO and import it to a 7.13+ instance (or master)
5. Check the datarange mode, now is set to timerange mode

Expected behavior:
The datarange mode should be the last_value and not the entire_time_range.

I think that the fix is to update the existing migration script to also take under consideration this case.

[elasticmachine]

Pinging @elastic/kibana-vis-editors (Team:VisEditors)

[VladLasitsa]

@stratoula we haven't created migration script for changing default mode to time range. We just use this logic: https://github.com/elastic/kibana/pull/93608/files#diff-50942ed6634e588d7217387986ad5972804cb19b28affb2b4e39cd5161535eb4R73

As I understand we should enhance this logic for this case?

[stratoula]

@VladLasitsa yes you are right :) I got confused with the drop_last_bucket one!